1. Technical Field of the Invention
The present invention relates to an electronic keyboard instrument which contains an electroacoustic transducer.
2. Description of the Related Art
The trend now is to shorten a size in a depth direction of an electronic keyboard instrument. However, an electronic circuit substrate unit equipped with a CPU and a semiconductor integrated circuit for performing key-press detection, sound generation, automatic musical performance and key-press guide, a power circuit unit which supplies power to the electronic circuit substrate unit, and an electroacoustic transducer such as a speaker unit which converts an electrical signal into sound, are typically mounted to a rear portion of a conventional keyboard musical instrument, because of their large bulk.
Because a speaker unit occupies a large mounting space, the speaker unit is the major impediment to size reduction and weight reduction of the musical instrument.
Particularly, in the piano-type keyboard instrument provided with a hammer (mass body), because a mass body pivoting mechanism has large bulk, it is more difficult to secure a space.
It is preferable to reduce a size of the speaker unit itself, however it is difficult to extend an output frequency band to a lower frequency range. Also, the speaker unit necessarily has a size of a certain extent, for example, a diameter of 5 cm or more in a circular or an elliptical shape.
There is a conventional electronic keyboard instrument containing a speaker unit, in which the speaker unit is mounted below a keyboard device while a front opening part of the speaker unit is directed downward, so that the sound is spread from a bottom surface of a case of the musical instrument (refer to Patent Reference 1). However, when the electronic keyboard instrument is placed on a desk, the sound is not spread well. To this end, in practical, the speaker unit cannot help being adopted to a cabinet type electronic keyboard instrument which has a sound emitting space below the speaker unit.
Also, there is another conventional electronic keyboard instrument, in which a large opening part is provided on a bottom plate of a case of the electronic keyboard instrument, and a speaker unit is mounted to a lower stand part, so that the sound from the speaker partially passes through a gap of keys of a keyboard device from the opening part, and is spread toward a player (refer to Patent Reference 2). However, because this electronic keyboard instrument is also a cabinet type, it has a large size.
[Patent Reference 1] Japanese Patent Laid-Open Publication No. 2003-15651
[Patent Reference 2] Japanese Patent Laid-Open Publication No. H7-325576